The present invention relates generally to plasma arc torches and, in particular, to consumable parts utilized in plasma arc torches and methods for improving the useful life of such consumable parts.
Plasma arc torches, also known as electric arc torches, are commonly used for cutting and welding metal workpieces by directing a plasma consisting of ionized gas particles toward the workpiece. In a typical plasma torch, a gas to be ionized is supplied to a lower end of the torch and flows past an electrode before exiting through an orifice in the torch tip. The electrode, which is a consumable part, has a relatively negative potential and operates as a cathode. The torch tip (nozzle) surrounds the electrode at the lower end of the torch in spaced relationship with the electrode and constitutes a relatively positive potential anode. The gas to be ionized typically flows through the chamber formed by the gap between the electrode and the tip in a generally swirling or spiraling flow pattern. When a sufficiently high voltage is applied to the electrode, an arc is caused to jump the gap between the electrode and the torch tip, thereby heating the gas and causing it to ionize. The ionized gas in the gap is blown out of the torch and appears as an arc that extends externally off the tip. As the head or lower end of the torch is moved to a position close to the workpiece, the arc jumps or transfers from the torch tip to the workpiece because the impedance of the workpiece to ground is made lower than the impedance of the torch tip to ground. During this “transferred arc” operation, the workpiece itself serves as the anode. A shield cap is typically secured on the torch body over the torch tip and electrode to complete assembly of the torch.
In addition to the electrode, other parts of the plasma arc torch are typically consumed during repeated operation of the torch, including the torch tip and the shield cap surrounding the tip. These consumable parts are consumed as a result of the destructive effects of the high heat environment, and effective management of the heat generated in and on these parts is critical to improving the useful life of the consumable parts. For example, heat is generated in the body of the electrode primarily by interaction with the heated plasma at its front face. Additional heat is generated in the electrode body by ohmic heating resulting from current flow. All of this heat in the electrode must be dissipated by conduction through the electrode body to a cooling mechanism.
To this end, it is known to provide a fluid cooled plasma arc torch in which the electrode is cooled primarily by high velocity plasma gas swirling through a plasma chamber formed by a gap between the electrode and surrounding tip. Plasma gas is directed over the outer surface of the electrode before it is ionized and exits through the tip orifice. A similar condition exists for the torch tip and the shield cap of a plasma arc torch. Heat developed in the tip and the shield cap is dissipated by convection to plasma gas flowing on the inside of the tip and by convection to secondary gas flowing on the outside of the tip. It is well established that cooling of the tip and the electrode during operation of the torch improves the useful life of these components.
Convective heat transfer (i.e., cooling) as discussed herein is the mechanism of heat removal in which heat in a body is deposited into fluid flowing over the surface of the body. The effectiveness of the cooling fluid flowing over the surface is referred to as the convective heat transfer coefficient h, which is impacted by velocity of the fluid flow, turbulence of the fluid flow, physical properties of the fluid, and interactions with surface geometry. In any convective cooling approach, a consequence of the fluid-surface interaction is the development of a region in the fluid adjacent to the surface, through which the fluid flow velocity varies from zero at the surface to a finite value associated with the bulk fluid flow near the center of the flow passage. This region is known as the hydrodynamic boundary layer. As illustrated in FIG. 13, in fully developed turbulent flow this boundary layer consists of three sublayers: a laminar sublayer adjacent the surface, an intermediate buffer layer and a turbulent outer layer. Heat transport across the laminar sublayer is dominated by conduction, while heat transport in the intermediate and turbulent layers is substantially augmented by the convective motion of the eddies present in these layers. The overall effect is that heat transfer from the surface to be cooled is substantially increased by the presence of turbulence in the boundary layer. Effective means for increasing convective heat transfer thus rely on increasing turbulence and mixing in the boundary layer, either by increasing the flow velocity or by promoting mixing or turbulence in the boundary layer as illustrated in FIG. 14.